Once you have the data, return to the Normandy and speak to Tali. She will ask to talk about the data you received. She wants a copy of it as her Pilgrimage gift. The only way to make her happy and give her something to return to her people with will be to give her a copy of the data, but it's your call.
Mass Effect 2 Consequences Edit
On Freedom's Progress, Shepard can verify his/her identity for Tali by mentioning the geth data that she received. Tali will confirm that it aided in her Pilgrimage. If the data was withheld, Shepard cannot reassure her in this manner, though Tali will not remain overly suspicious. In this case, her Pilgrimage is still presumably a success, though it is unknown what she used as a substitute gift.